Techniques for network protection based on subscriber-aware application proxies

ABSTRACT

Techniques for responding to intrusions on a packet switched network include receiving user data at a subscriber-aware gateway server between a network access server and a content server. The user data includes subscriber identifier data that indicates a unique identifier for a particular user, network address data that indicates a network address for a host used by the particular user, NAS data that indicates an identifier for the network access server, flow list data that indicates one or more open data packet flows, and suspicious activity data. The suspicious activity data indicates a value for a property of the open data packet flows that indicates suspicious activity. It is determined whether an intrusion condition is satisfied based on the suspicious activity data. If the intrusion condition is satisfied, then the gateway responds based at least in part on user data other than the network address data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to protection from attacks and fraud in acommunication network with application proxies that are subscriberaware, such as service gateways that regulate application activity; and,in particular, to discerning a user identifier other than networkaddress of a user initiating an attack or fraud in order to end theintrusion, such as a scanning attack that initiates packet flows to alarge number of destinations in a short time.

2. Description of the Related Art

Networks of general-purpose computer systems and other devices connectedby external communication links are well known. The networks ofteninclude one or more network devices that facilitate the passage ofinformation between the computer systems. A network node is a networkdevice or computer system connected by the communication links. As usedherein, an end node is a network node that is configured to originate orterminate communications over the network. In contrast, an intermediatenetwork node facilitates the passage of data between end nodes.

Information is exchanged between network nodes according to one or moreof many well known, new or still developing protocols. In this context,a protocol consists of a set of rules defining how the nodes interactwith each other based on information sent over the communication links.The protocols are effective at different layers of operation within eachnode, from generating and receiving physical signals of various types,to selecting a link for transferring those signals, to the format ofinformation indicated by those signals, to identifying which softwareapplication executing on a computer system sends or receives theinformation. The conceptually different layers of protocols forexchanging information over a network are described in the Open SystemsInterconnection (OSI) Reference Model. The OSI Reference Model isgenerally described in more detail in Section 1.1 of the reference bookentitled Interconnections Second Edition, by Radia Perlman, publishedSeptember 1999, which is hereby incorporated by reference as thoughfully set forth herein.

Communications between nodes are typically effected by exchangingdiscrete packets of data. Each packet typically comprises 1] headerinformation associated with a particular protocol, and 2] payloadinformation that follows the header information and contains informationthat may be processed independently of that particular protocol. In someprotocols, the packet includes 3] trailer information following thepayload and indicating the end of the payload information. The headerincludes information such as the source of the packet, its destination,the length of the payload, and other properties used by the protocol.Often, the data in the payload for the particular protocol includes aheader and payload for a different protocol associated with a different,typically higher layer of the OSI Reference Model. The header for aparticular protocol typically indicates a type for the next protocolcontained in its payload. The higher layer protocol is said to beencapsulated in the lower layer protocol. The headers included in apacket traversing multiple heterogeneous networks, such as the Internet,typically include a physical (layer 1) header, a data-link (layer 2)header, an internetwork (layer 3) header, and some combination of atransport (layer 4) header, a session (layer 5) header, a presentation(layer 6) header and an application (layer 7) header as defined by theOpen Systems Interconnection (OSI) Reference Model. In networkingparlance, a tunnel for data is simply a protocol that encapsulates thatdata.

Subscribers obtain access to a packet-switched network (PSN) of anInternet Service Provider (ISP) through a Network Access Server (NAS). Asubscriber often uses a link-layer protocol to form a temporary tunnelbetween the subscriber's device and the NAS. The contents of thetunneling protocol payload are not involved in determining the path. TheNAS determines whether an entity attempting access is in fact asubscriber authorized to access the network by exchanging packets withan Authentication, Authorization, and Accounting (AAA) server. Examplewell-known AAA servers include the Remote Authentication Dial In UserService (RADIUS) server, Terminal Access Controller Access ControlSystem (TACACS), and the DIAMETER server. Once the entity isauthenticated to be an authorized subscriber, then access is granted tothe ISP network, the subscriber is assigned a network layer address,such as an Internet Protocol (IP) address, and internetwork-layerpayloads are routed based on the internetwork and higher layer headerinformation.

A modem ISP can offer different services to different subscribers,including services delivered in protocol layers 4 through 7. Forexample, the rate of data delivery of large Web pages to somesubscribers can be increased by compressing the Web pages beforedelivery and un-compressing the Web pages at a process on thesubscriber's own equipment. As is well known in the art, Web pages aretransmitted over a network using the Hypertext Transfer Protocol (HTTP),an application-layer (layer 7) protocol. Certain Web pages can beblocked using a Web filtering service. A service that provides somecombination of compression, filtering and local caching of Web pages iscalled Web optimization. Some subscribers use mobile devices, such ascell phones, that have smaller memory and display capacities than othernetwork devices. Web pages are communicated to such mobile devices usingspecial protocols, such as the Wireless Application Protocol (WAP), anapplication-layer protocol. HTTP payloads are translated to WAP payloadsbefore delivery to these subscribers.

To deliver these special services, service gateways are included in theISP packet switched networks. Service gateways are processes thatoperate on intermediate network devices between the source and thedestination of data packets. The service gateways inspect packetpayloads for the purpose of delivering a network service. Exampleservices include payload translation, just described, and other payloadchanges, as well as special billing, rating, filtering services andother services that do not modify the contents of a payload. Forexample, Web compression gateways compress HTTP payloads of data packetsdirected to a subscriber's device and un-compress HTTP payloads of datapackets originating from a subscriber's device. A WAP 1.x gatewayconverts HTTP payloads of data packets directed to a subscriber's deviceto WAP 1.x payloads and converts WAP 1.x payloads of data packetsoriginating from a subscriber's device to HTTP payloads. Some ISPs offerdifferent services to different subscribers. These are subscriber-awareservices.

To ensure that a service gateway for a service offered by the ISP isincluded in packet-switched paths from the subscriber to any destinationon the network accessed by the ISP network, the service gateway isincluded as a proxy for an actual destination used to set up asubscriber's session on the network. For example, AAA server traffic fora NAS is directed to a service gateway, which serves as a proxy for theAAA server. A subscriber-aware service gateway monitors the AAA servertraffic to determine the remote user's network identifier and whetherthe remote user has subscribed to the service provided by the gateway.For example, the service gateway monitors RADIUS to determine mapping ofsubscriber ID to currently assigned network ID; and, in addition, RADIUSis used to relay information on users' subscribed service profile to thenetwork elements from a back-end database, typically behind the RADIUSserver.

It is common for an ISP to include a cluster of service gateways so theservice can be scaled to the number of subscribers. To distributetraffic among the service gateways in the cluster, a load balancerprocess is included in the path between the NAS (or other end node) andthe cluster of service gateways.

It is also common for an ISP to include a firewall server in the pathbetween the NAS and the service gateway (or its load balancer process).The firewall determines whether data packets are received from anunwanted source or directed to an unwanted destination on the accessnetwork and does not forward such data packets. The unwanted sources anddestinations are identified by their IP addresses. The IP addresses ofunwanted sources and destinations are determined by a policy indicatedby policy data at the firewall. The policy may be based on static listsof IP addresses, or dynamically determined IP addresses based on one ormore characteristics of a flow of data packets.

A flow of data packets is a series of one or more data packets within areasonable period of time from the same source process to the samedestination process on a network. The source and destination processesare typically identified based on some combination of their layer 3 IPaddresses and layer 4 transport ports. Unwanted IP addresses may bedefined in any manner, including static lists of one or more IPaddresses, and methods for dynamically determining one or more unwantedIP addresses based on characteristics of one or more flows with that IPaddress. Intermediate network node operating systems, such as theInternetwork Operating System (IOS) of Cisco Systems, San Jose, Calif.,define Access Control Lists (ACL) for identifying and filtering unwantedIP addresses.

While suitable for many purposes, there are some deficiencies with theprior approaches that use firewalls to exclude unwanted traffic. Onedeficiency is that firewalls are sometimes not deployed by customersbecause the firewalls add to the cost of a network and can diminish theperceived performance. Thus firewalls are sometimes not deployed,leaving the service gateways vulnerable to attacks in application layers4 through layer 7. For example, in a scan attack, a malicious processrunning on an end node can initiate traffic to a large number of IPaddress (layer 3) and port (layer 4) destination combinations on thetarget network within a short period of time. Resources at the servicegateway and beyond are consumed in processing individual transactionswithin each such flow to engage the correct server for the correspondingsubscriber. If a sufficient number of such flows are initiated, theservice gateway or its cluster can become so encumbered that legitimateflows are not processed in a timely manner or, in some cases, at all.

Another deficiency is that a firewall alerted to the IP address of sucha malicious end node only protects against traffic from that same IPaddress. The actual subscriber using the device with the unwanted IPaddress is not identified. Thus if the same subscriber switches theattack to launch from another device with a different IP address, suchas by moving with a cell phone to a different point in the accessnetwork or to a different access network, the firewall has to rediscoverthe new unwanted IP address. In the meantime the service gateway andaccess network have both wasted resources on the data flows from thesame malicious subscriber at the new IP address.

Another deficiency is that a firewall alerted to the IP address of sucha malicious end node only protects the target network downstream fromthe firewall. Thus malicious traffic from a user on the access networkthat is detected by a firewall on the target network protects the targetnetwork but not the access network. Considerable access networkresources upstream of the firewall are consumed by the attack only to bedropped by the firewall in protecting the target network.

Another disadvantage is that firewalls do not detect fraud. Whereas anattack is typically aimed at openly degrading the performance ofequipment of the ISP; fraud is typically aimed at undetectably runningnon-permitted protocols/application on the network without. The firewallis often unable to determine whether a reasonable amount of trafficrepresents an unauthorized use of privileged or restricted protocols orapplications.

Based on the foregoing description, there is a clear need for protectionfrom attacks and fraud (called hereinafter “intrusions”) that consumesignificant network resources for service gateway processes, whichprotection does not suffer all the deficiencies of prior art approaches.In particular, there is a need for techniques that detect intrusions inlayer 4 through layer 7 protocols and either identifies users ofmalicious end nodes by name or detaches such malicious end nodes fromthe access network to free up resources in the access network, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a block diagram that illustrates a remote access network thatincludes a service gateway server, according to an embodiment;

FIG. 2 is a block diagram that illustrates table of subscriberinformation on a service gateway server, according to an embodiment;

FIG. 3 is a block diagram that illustrates a method at a high level forprotecting against attacks at a service gateway, according to anembodiment;

FIG. 4 is a block diagram that illustrates a method at a high level forprotecting against attacks at a billing agent, according to anembodiment; and

FIG. 5 is a block diagram that illustrates a computer system, such as arouter, upon which an embodiment of the invention may be implemented.

DETAILED DESCRIPTION

Techniques are described for protecting against intrusions (includingeither or both fraud and attacks) at a service gateway server. In thefollowing description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form in order to avoidunnecessarily obscuring the present invention.

Embodiments of the invention are illustrated with many examples in thecontext of a NAS gateway, but the invention is not limited to thiscontext. In other embodiments other service gateways are used to provideprotection from network intrusions, such as WAP gateways and othersubscriber-aware gateways.

1.0 Network Overview

FIG. 1 is a block diagram that illustrates a remote access network 100that includes network access servers (NAS) 125 and service gatewayserver 160, according to an embodiment. A computer network is ageographically distributed collection of interconnected sub-networks(e.g., sub-networks 110 a, 110 b, collectively referenced hereinafter assub-networks 110) for transporting data between nodes, such ascomputers, personal data assistants (PDAs) and cell phones. Eachsub-network itself may be composed of one or more sub-networks. A localarea network (LAN) is an example of a sub-network. The network'stopology is defined by an arrangement of end nodes (e.g., end nodes 120a, 120 b, collectively referenced hereinafter as end nodes 120) thatcommunicate with one another, typically through one or more intermediatenetwork nodes, such as a router or switch, that facilitates routing databetween end nodes 120 on different sub-networks. Two sub-networks 110that are typically involved in remote access are depicted in FIG. 1.Each sub-network 110 may include zero or more intermediate networknodes. An Internet Protocol (IP) packet-switched network (PSN) 110 b isthe target for remote access by users of end nodes 120 a, 120 b at oneor more remote sites. The remote sites are connected to the PSN 110 bthrough an access network 110 a.

In various embodiments, access sub-network 110 a is built, at least inpart, on a telephone twisted pair, coaxial copper, optical cable or awireless infrastructure. In various embodiments, access network 110 aincludes a controller for a bank of low-bandwidth modems, a digitalsubscription line (DSL) access module (DSLAM), or other coaxial cable oroptical access modules. Although two end nodes 120 a, 120 b are depictedconnected to access network 110 a, in other embodiments more or fewerend nodes are connected to access network 110 a. For mobile devices, theaccess network 110 a includes circuitry and logic to maintaincommunication as the mobile device moves from one wireless access point(e.g., a mobile telephone cell antenna) to another.

Communications over sub-network 110 a from end nodes 120 a, 120 bterminate at network access server (NAS) 125 a executing on one or morehosts. Other end nodes using other access networks terminate at otherNAS, such as NAS 125 b on another host or hosts. Although two NASs 125a, 125 b are depicted connected to PSN 110 b, in other embodiments moreor fewer NASs are connected to PSN 110 b. In various embodiments the NASis a Broadband Remote Access Server (BRAS), a wireless access server, orsome other server.

The client-server model of computer process interaction is widely knownand used in commerce. According to the client-server model, a clientprocess sends a message including a request to a server process, and theserver process responds by providing a service. The server process mayalso return a message with a response to the client process. Often theclient process and server process execute on different computer or othercommunicating devices, called hosts, and communicate via a network usingone or more protocols for network communications. The term “server” isconventionally used to refer to the process that provides the service,or the host computer on which the process operates. Similarly, the term“client” is conventionally used to refer to the process that makes therequest, or the host computer on which the process operates. As usedherein, the terms “client” and “server” refer to the processes, ratherthan the host computers, unless otherwise clear from the context. Inaddition, the process performed by a server can be broken up to run asmultiple servers on multiple hosts (sometimes called tiers) for reasonsthat include reliability, scalability, and redundancy, but not limitedto those reasons.

PSN 110 b is used by end nodes 120 a, 120 b at a remote site tocommunicate with servers, such as servers 170 a, 170 b, 170 c(collectively referenced hereinafter as server 170). PSN 110 b includesAAA server 114, to authenticate the user of end nodes 120 attempting toaccess PSN 110 b through any NAS 125. Only a subscriber registered withthe ISP and listed in the AAA server 114 is given access to PSN 110 b.PSN 110 b also includes billing agent server 116, to collect informationabout use of network 110 b by a particular subscriber for purposes ofobtaining payment from that subscriber. In an illustrated embodiment,billing agent 116 determines when and whether a subscriber is to belisted with the AAA server 114. An example billing agent server is aBilling Mediation Agent (BMA). In some embodiments, the billing agent116 is omitted and the functions ascribed thereto in the following aredistributed among other servers, such as AAA server 114.

To provide special subscriber-aware services, PSN 110 b includes servicegateway 160. In some embodiments service gateway 160 includes a clusterof multiple service gateways and load balancers (not shown). Althoughone service gateway 160 is shown in FIG. 1 for purposes of illustration,in other embodiments PSN 110 b includes more service gateways 160.

To provide subscriber-aware services, the service gateway 160 isinserted in the path from NAS 125 to AAA server 114, to determine whatsubscriber is associated with a particular address on network 110 b.That is, the service gateway server 160 advertises itself as resolvingtraffic directed to AAA server 114. The service gateway server 160snoops through the AAA messages exchanged between the NAS 125 and theAAA server 114, to determine if an IP address is associated with asubscriber for the service provided by the service gateway server 160.The service gateway 160 is also inserted in the path from NAS 125 toanother server on network 110 b, e.g., in paths to servers 170 a, 170 b,170 c. For example, all traffic to NAS is routed to service gateway 160.

2.0 Table of Subscriber Information

FIG. 2 is a block diagram that illustrates a table of subscriberinformation on a service gateway server according to an embodiment.Although data fields for holding data in table 260 are displayed in anintegral data structure in FIG. 2A for purposes of illustration, inother embodiments, one or more fields or portions of fields are held inany sequence in volatile or non-volatile memory in one or more differentdata structures in the same or different hosts and controlled by zero ormore different data servers, such as one or more database servers.

Table 260 includes subscriber record fields 270 a, 270 b and othersindicated by ellipsis 290 (collectively referenced hereinafter assubscriber records 270). Each subscriber record 270 includes fields forsubscriber identifier, network address, network access server (NAS)information, subscriber profile information, a list of open flows, as istypical of current service gateway servers. According to the illustratedembodiments of the invention, Table 260 also includes a measure ofsuspicious activity based on the open flows, as described in more detailbelow. These subscriber records associate a particular subscriber withsuspicious activity in one or more open data flows.

An open data flow is a series of one or more data packets that beginwith an initial data packet, such as a synchronization (SYN) messageaccording to a protocol at layer 4 or higher, and that does not includea corresponding termination data packet, such as a finished (FIN)message, according to that protocol. In various embodiments, layer 3source and destination addresses (e.g., source and destination IPaddresses) with or without a layer 4 source or destination address(e.g., source and destination TCP ports) or other data fields in layer 3to layer 7 headers are used to distinguish one data packet flow fromanother at the service gateway server.

Subscriber records 270 a, 270 b include subscriber ID fields 272 a, 272b, respectively (collectively referenced hereinafter as subscriber IDfield 272), which hold data that uniquely identifies a particularsubscriber to network services on the target network, e.g., PSN 110 b,who has at least one open data flow. In some embodiments, a subscriberrecord 270 is deleted from table 260 when the subscriber no longermaintains at least one open flow. Any identifier for a subscriber knownin the art may be used as the identifier in subscriber ID field 272. Forexample, a subscriber identifier determined by Network Access Identifier(NAI) is used in some embodiments (see RFC2486, the entire contents ofwhich are hereby incorporated by reference as if fully set forthherein). For example, a subscriber ID field 272 a holds data thatindicates “Alice@companyA.com.”

Subscriber records 270 a, 270 b also include network address fields 274a, 274 b, respectively (collectively referenced hereinafter as networkaddress field 274), which hold data that indicates a network address fora network node (e.g., end node 120 b) from which traffic originates fromthe particular subscriber identified in the subscriber ID field. Forexample, in some embodiments the network address field 274 includes aTCP port (layer 4 source), an IP address (layer 3 source), a virtualprivate network (VPN) identifier (layer 3 or layer 2 subset of sources),or an Ethernet virtual local area network (VLAN) tag (layer 2 subset ofsources), or some combination.

Subscriber records 270 a, 270 b also include NAS information fields 276a, 276 b, respectively (collectively referenced hereinafter as NASinformation field 276), which hold data that indicates information abouta particular NAS (e.g., NAS 125 a) through which traffic from theparticular subscriber enters the target network (e.g., PSN 110 b). Forexample, in some embodiments the NAS information field 276 holds datathat indicates an IP address of the NAS. In some embodiments the NASinformation field 276 holds data that indicates an NAS identifierassigned according to NAI or some other domain name server (DNS).

Subscriber records 270 a, 270 b also include subscriber profileinformation fields 278 a, 278 b, respectively (collectively referencedhereinafter as subscriber profile information field 278), which holddata that indicates information about the particular user's access tothe target network. For example, in some embodiments the subscriberprofile information field 278 holds data that indicates policies forpassing or blocking traffic from the particular subscriber, a quota ofuses for the network, whether the subscriber is paid up or has pre-paidfor one or more services, or some combination.

Subscriber records 270 a, 270 b also include list of open flows fields280 a, 280 b, respectively (collectively referenced hereinafter as openflow list 274), which hold data that indicates a number of flows and foreach flow a unique identifier for the data packet flow. A uniqueidentifier for a data packet flow can be defined in any manner known inthe art. In an example embodiment, the identifier for the flow isconstructed from a network address for a destination of traffic from theparticular user's network address as indicated by the contents ofnetwork address field 274 a, described above. For example, in someembodiments the unique flow identifier for each flow in the open flowslist 280 includes a TCP port (layer 4 destination), an IP address (layer3 destinations), a virtual private network (VPN) identifier (layer 3 orlayer 2 subset of destinations), or an Ethernet virtual local areanetwork (VLAN) tag (layer 2 subset of destinations), or somecombination.

According to the illustrated embodiments, subscriber records 270 a, 270b also include measure of suspicious activity fields 282 a, 282 b,respectively (collectively referenced hereinafter as suspicious activityfield 282), which hold data that indicates a measure of suspiciousactivity based at least in part on the data flows indicated by the openflows list 280. Any method known in the art for detecting hostileactivity by a user may be used to determine the measure of suspiciousactivity that is stored in field 282, as described in more detail belowwith reference to FIG. 3. For example, in some embodiments, the measureof suspicious activity stored in field 282 is the number of open flowsestablished over a particular time period, such as the most recentinterval of time equal to the particular time period. As is described inmore detail below, an excessive number of open flows established in theparticular period of time is determined to be a scanning attack.

3.0 Method at Service Gateway Server

FIG. 3 is a block diagram that illustrates a method 300 at a high levelfor protecting against intrusions at a service gateway server. Althoughsteps are shown in FIG. 3 and subsequent flow diagrams in a particularorder for purposes of illustration, in other embodiments one or moresteps are performed in a different order or overlapping in time or oneor more steps are omitted or the steps are changed in some combinationof ways.

In step 310, subscriber data is received. The subscriber data indicatesfor a particular subscriber all open data flows associated with thatsubscriber and a particular property of the open data flows that is ameasure of suspicious activity. For example, the data for Table 260 isreceived during step 310. Any method known in the art to receive thisinformation may be used. For example, the data may be received either inresponse to one or more queries or unsolicited from a local or remotevolatile or permanent storage controlled directly by the gateway serveror indirectly by one or more database or other servers.

Current service gateway servers already receive and store data for thesubscriber ID field 272, the network address field 274, the NASinformation field 276, the subscriber profile information field 278, andthe open flows list 280. According to illustrated embodiments of theinvention, data is received for the measure of suspicious activity field282 as well.

In the illustrated embodiment, the measure of suspicious activity is thenumber of open flows established within the particular period of time.For example, a counter is initialized at a time a first open flow isestablished and incremented with each new open flow established withinone second of the first flow.

In a scanning attack, a process associated with a subscriber opens manyflows in a short time in order to consume resources at the servicegateway server and reduce the ability of the service gateway server toprocess all the traffic from a particular NAS, thus denying service toone or more other subscribers. Such an attack is characterized by alarge number of open flows established per second. It has been estimatedthat a laptop computer available at the time of this writing cangenerate about 200,000 SYN messages that open new data flows per secondand itself launch a damaging scanning attack. Such an attack can bedetected by counting the number of data flows opened per second.

Any indication of suspicious activity associated with an intrusion onnetwork resources known in the art may be used as the measure ofsuspicious activity, including measures of suspicious activity used infirewall processes. In some embodiments the measure of suspiciousactivity is a number or a percentage of suspicious open data flows.

In some embodiments, an open data flow is considered suspicious if aninitial SYN message is not followed by an acknowledgement from thetargeted server in a SYN/ACK message. This is a measure of suspiciousactivity because, in some scanning attacks, the destination of the SYNmessages is a non-existent server or random IP and port address forwhich there is no server to respond with a SYN/ACK message. Thus thenumber of open flows for which no SYN/ACK is received is a measure ofsuspicious activity.

In some embodiments, an open data flow is considered suspicious if aninitial SYN message followed by an acknowledgement from the targetedserver in a SYN/ACK message is not then followed by an acknowledgementfrom the subscriber in an ACK message. This is a measure of suspiciousactivity because, in some scanning attacks, the destination of the SYNmessages is often a real server which produces a SYN/ACK message; butthe attacker is interested only in establishing a new flow to consumegateway server resources and is not interested in continuing the sessionwith the server. Thus the attacking node of the particular subscriberdoes not respond with an ACK message.

Other measures of suspicious activity are used in other embodiments,such as other attacks and fraud. For example, to detect the use ofbanned applications, a layer 7 service gateway detects the request forservice from the banned application and notes it as fraud. The number ofcalls for the banned application in such embodiments is a measure ofsuspicious activity. The most egregious users would be the target of anyresponse. In another example, total bandwidth consumed by a user duringa particular time interval is a measure of suspicious activity.Generally a user subscribes to a maximum amount of bandwidth, e.g., 10¹⁰bits (10 Gigabits) per month. Bandwidth usage above this amount is notedas fraud. With subscriber awareness, service gateway server (e.g.,server 160) can count the total bandwidth used across multiple logins bythe same subscriber. Such activity is not detected by a firewall.

In step 330, it is determined whether there is an intrusion conditionbased on the measure of suspicious activity. For example, in someembodiments, it is determined that there is an intrusion condition ifthe number of data flows opened per second, stored in the measure ofsuspicious activity field 282, exceeds 100. In some embodiments, it isdetermined that there is an intrusion condition if the number ofsuspicious open data flows, stored in the measure of suspicious activityfield 282, exceeds 75. In some embodiments, it is determined that thereis an intrusion condition if the number of suspicious open data flows,stored in the measure of suspicious activity field 282, divided by thenumber of open flows stored in the list of open flows field 280 exceeds50%. In some embodiments, it is determined that there is an intrusioncondition if the number of requests for a banned or limited application,exceeds 1. In some embodiments, it is determined that there is anintrusion condition if the bandwidth used per month exceeds 10¹⁰Gigabits. In other embodiments, other intrusion conditions are used andtested during step 330.

If it is determined in step 330 that there is not an intrusion conditionbased on the measure of suspicious activity, control passes back to step310 to continue receiving data about subscribers and their open flows orrequested applications or total bandwidth, or some combination.

If it is determined in step 330 that there is an intrusion conditionbased on the measure of suspicious activity, control passes to step 350to respond to the intrusion based on the particular subscriber who isassociated with the measure of suspicious activity that satisfies theintrusion condition. Any response that utilizes the subscriberinformation may be performed. In other embodiments, any response notperformed by a firewall is performed, such as a response based on anysubscriber data other than the IP address of the attacker.

In the illustrated embodiment, the response includes steps 352, 354,356. In other embodiments one or more of these steps are omitted, one ormore other steps are included, or some combination of omitted and otheradded steps is employed.

In step 352, a message that identifies the subscriber is sent to abilling agent to penalize the subscriber. For example, a message is sentto the billing agent with a subscriber identifier based on the contentsof subscriber ID field 272. The billing agent then causes the subscriberto be penalized, as described below with reference to FIG. 4. Possiblepenalties includes removal of authorization for the subscriber to accessthe network, charging the subscriber an excess fee, and sending an“out-of-band” message to the subscriber outside data network 110 b. Anout-of-band messages includes a text message or voice call to a cellphone or other telephone or an email to an email address or a letter bypost. The out-of-band message alerts the subscriber to the intrusion andrequests that the subscriber cease the intrusion. By virtue of step 352,the subscriber who launches the attack or fraud is penalized in moreways than simply dropping data packets originating from the subscriber'sIP address. Thus step 552 provides superior protection than thatprovided by a firewall.

In step 354, a message is sent to the NAS associated with the subscriberto terminate communications with the subscriber. Any method may be usedto cause the NAS to terminate communications. In an illustratedembodiment, a RADIUS Packet of Disconnect (POD) message is sent to theNAS identifying the subscriber by ID or IP address or both. The RADIUSpacket of Disconnect is described in RFC 3576, the entire contents ofwhich are hereby incorporated by reference as if fully set forth herein.According to RFC 3576, “unsolicited messages sent from the RADIUS serverto the NAS [are supported]. These extended commands provide support forDisconnect and Change-of-Authorization (CoA) messages. Disconnectmessages cause a user session to be terminated immediately, whereas CoAmessages modify session authorization attributes such as data filters.”When the NAS receives the POD, it tears down the call from the end nodethrough the access network to the NAS, thus freeing network resources inthe access network. By virtue of step 354, in communications directed tothe target network, the access network upstream of the service gatewayis also protected from attacks. Thus, step 354 provides superiorprotection than that provided by a firewall.

In step 356, the subscriber is prevented from logging on to the targetnetwork for a particular penalty period of time. In other embodiments,the subscriber is prevented from logging on to the target networkindefinitely, until a network manager intervenes to restore networkprivileges to the subscriber. Any method may be used to prevent loggingon. For example, the gateway server 160 snoops all logon messagesdirected to the AAA server 114 and drops those messages that indicatethe subscriber associated with the suspicious activity, or sends areturn message indicating logon failed. In some embodiments, a messageis sent to the AAA server 114 or billing agent server 116 to removeauthorization for the subscriber. In some embodiments, a message is sentto the AAA server 114 or billing agent server 116 to restoreauthorization for the subscriber after the penalty period expires. Byvirtue of step 356, the subscriber is unable to launce attacks fromanother device. Thus step 356 provides superior protection than thatprovided by a firewall.

In some embodiments, during step 356, the logon request is sent to aspecial repair server, which responds by notifying the subscriber of theattack or fraud launched by the subscriber's device and offers to repairthe subscriber's device so that it does not launch the intrusion again.If the subscriber agrees, software is sent to be executed on thesubscriber's device to find and remove the software that causes thesubscriber's device to launch the intrusion. In this way an attackinitiated by a virus that installed software on an unsuspectingsubscriber's device, without the knowledge of the subscriber, does notcause the subscriber to be unduly penalized.

A service gateway server is a superior network node for protecting thenetwork 100 from attacks on layer 4 through layer 7 network resources.The service gateway server (e.g., gateway 160) is the only network nodein the illustrated embodiment that both tracks activity at layers 4through layer 7 while also maintaining a mapping between a usercurrently assigned an IP address and a corresponding subscriber's name(e.g., a “user name”). This allows the service gateway server (e.g.,gateway server 160) to generate a detailed account of the maliciousactivity, including network ID and user name.

4.0 Method at Billing Agent Server

FIG. 4 is a block diagram that illustrates a method 400 at a high levelfor protecting against intrusions at a billing agent, according to anembodiment. In other embodiments, one or more steps of method 400 areomitted.

In step 410, malicious subscriber data is received. This data indicatesa particular subscriber associated with an intrusion detected by aservice gateway server. In the illustrated embodiment, the messageincludes data that indicates the subscriber ID (e.g., a “user name”) anddata that indicates that an intrusion is associated with thissubscriber. In some embodiment, the data further indicates one or morespecific details about the particular intrusion. In the illustratedembodiment, the data is received at billing agent 116 from the servicegateway server 160 in an unsolicited message. In other embodiments, thedata is received in other ways, e.g., by querying data stored locally orremotely in volatile or permanent storage accessed directly orindirectly through a database server. For example, during the course ofa duty cycle or upon receipt of an alert, the billing agent retrievesdata from storage that indicates the subscriber ID of a particularsubscriber and an indication of an associated intrusion.

In step 420, a message is sent to the AAA server to remove theparticular subscriber from the list of authorized users of the targetnetwork (e.g., PSN 110 b). In an illustrated embodiment, the subscriberID is included in the message to the AAA server.

In step 424, an out-of-band message is sent to the particular subscriberindicating that the subscriber should cease the attack.

In step 426, an excess cost is charged to the account of the particularsubscriber as a penalty for the attack.

In step 430, it is determined whether a penalty period has expired. Apenalty period is the duration of time between a time when a subscriberis removed from authorization to access the target network and a timewhen the subscriber is again authorized to access the target network. Ifit is determined that the penalty period has not expired, then controlpasses to step 436. In step 436, the billing agent continues with itsnormal processing and returns periodically to step 430 to determine ifthe penalty period has yet expired.

If it is determined during step 430 that the penalty period has expired,then control passes to step 440. In step 440, a message is sent to theAAA server to restore the particular subscriber to the list ofauthorized users of the target network (e.g., PSN 110 b). In anillustrated embodiment, the subscriber ID is included in the message tothe AAA server.

In embodiments in which the malicious subscriber is prevented fromaccessing the network indefinitely, steps 430 and 440 are omitted.

5.0 Implementation Mechnisms—Hardware Overview

FIG. 5 is a block diagram that illustrates a computer system 500 uponwhich an embodiment of the invention may be implemented. The preferredembodiment is implemented using one or more computer programs running ona network element such as a router device. Thus, in this embodiment, thecomputer system 500 is a router.

Computer system 500 includes a communication mechanism, such as a bus510 for passing information between other internal and externalcomponents of the computer system 500. Information is represented asphysical signals of a measurable phenomenon, typically electricvoltages, but including, in other embodiments, such phenomena asmagnetic, electromagnetic, pressure, chemical, molecular atomic andquantum interactions. For example, north and south magnetic fields, or azero and non-zero electric voltage, represent two states (0, 1) of abinary digit (bit). A sequence of binary digits constitutes digital datathat is used to represent a number or code for a character. A bus 510includes many parallel conductors of information so that information istransferred quickly among devices coupled to the bus 510. One or moreprocessors 502 for processing information are coupled with the bus 510.A processor 502 performs a set of operations on information. The set ofoperations include bringing information in from the bus 510 and placinginformation on the bus 510. The set of operations also typically includecomparing two or more units of information, shifting positions of unitsof information, and combining two or more units of information, such asby addition or multiplication. A sequence of operations to be executedby the processor 502 constitute computer instructions.

Computer system 500 also includes a memory 504 coupled to bus 510. Thememory 504, such as a random access memory (RAM) or other dynamicstorage device, stores information including computer instructions.Dynamic memory allows information stored therein to be changed by thecomputer system 500. RAM allows a unit of information stored at alocation called a memory address to be stored and retrievedindependently of information at neighboring addresses. The memory 504 isalso used by the processor 502 to store temporary values duringexecution of computer instructions. The computer system 500 alsoincludes a read only memory (ROM) 506 or other static storage devicecoupled to the bus 510 for storing static information, includinginstructions, that is not changed by the computer system 500. Alsocoupled to bus 510 is a non-volatile (persistent) storage device 508,such as a magnetic disk or optical disk, for storing information,including instructions, that persists even when the computer system 500is turned off or otherwise loses power.

The term computer-readable medium is used herein to refer to any mediumthat participates in providing information to processor 502, includinginstructions for execution. Such a medium may take many forms,including, but not limited to, non-volatile media, volatile media andtransmission media. Non-volatile media include, for example, optical ormagnetic disks, such as storage device 508. Volatile media include, forexample, dynamic memory 504. Transmission media include, for example,coaxial cables, copper wire, fiber optic cables, and waves that travelthrough space without wires or cables, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared waves.Signals that are transmitted over transmission media are herein calledcarrier waves.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, a hard disk, a magnetic tape or any othermagnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD)or any other optical medium, punch cards, paper tape, or any otherphysical medium with patterns of holes, a RAM, a programmable ROM(PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memorychip or cartridge, a carrier wave, or any other medium from which acomputer can read.

Information, including instructions, is provided to the bus 510 for useby the processor from an external terminal 512, such as a terminal witha keyboard containing alphanumeric keys operated by a human user, or asensor. A sensor detects conditions in its vicinity and transforms thosedetections into signals compatible with the signals used to representinformation in computer system 500. Other external components ofterminal 512 coupled to bus 510, used primarily for interacting withhumans, include a display device, such as a cathode ray tube (CRT) or aliquid crystal display (LCD) or a plasma screen, for presenting images,and a pointing device, such as a mouse or a trackball or cursordirection keys, for controlling a position of a small cursor imagepresented on the display and issuing commands associated with graphicalelements presented on the display of terminal 512. In some embodiments,terminal 512 is omitted.

Computer system 500 also includes one or more instances of acommunications interface 570 coupled to bus 510. Communication interface570 provides a two-way communication coupling to a variety of externaldevices that operate with their own processors, such as printers,scanners, external disks, and terminal 512. Firmware or software runningin the computer system 500 provides a terminal interface orcharacter-based command interface so that external commands can be givento the computer system. For example, communication interface 570 may bea parallel port or a serial port such as an RS-232 or RS-422 interface,or a universal serial bus (USB) port on a personal computer. In someembodiments, communications interface 570 is an integrated servicesdigital network (ISDN) card or a digital subscriber line (DSL) card or atelephone modem that provides an information communication connection toa corresponding type of telephone line. In some embodiments, acommunication interface 570 is a cable modem that converts signals onbus 510 into signals for a communication connection over a coaxial cableor into optical signals for a communication connection over a fiberoptic cable. As another example, communications interface 570 may be alocal area network (LAN) card to provide a data communication connectionto a compatible LAN, such as Ethernet. Wireless links may also beimplemented. For wireless links, the communications interface 570 sendsand receives electrical, acoustic or electromagnetic signals, includinginfrared and optical signals, which carry information streams, such asdigital data. Such signals are examples of carrier waves

In the illustrated embodiment, special purpose hardware, such as anapplication specific integrated circuit (IC) 520, is coupled to bus 510.The special purpose hardware is configured to perform operations notperformed by processor 502 quickly enough for special purposes. Examplesof application specific ICs include graphics accelerator cards forgenerating images for display, cryptographic boards for encrypting anddecrypting messages sent over a network, speech recognition, andinterfaces to special external devices, such as robotic arms and medicalscanning equipment that repeatedly perform some complex sequence ofoperations that are more efficiently implemented in hardware.

In the illustrated computer used as a router, the computer system 500includes switching system 530 as special purpose hardware for switchinginformation for flow over a network. Switching system 530 typicallyincludes multiple communications interfaces, such as communicationsinterface 570, for coupling to multiple other devices. In general, eachcoupling is with a network link 532 that is connected to another devicein or attached to a network, such as local network 580 in theillustrated embodiment, to which a variety of external devices withtheir own processors are connected. In some embodiments an inputinterface or an output interface or both are linked to each of one ormore external network elements. Although three network links 532 a, 532b, 532 c are included in network links 532 in the illustratedembodiment, in other embodiments, more or fewer links are connected toswitching system 530. Network links 532 typically provides informationcommunication through one or more networks to other devices that use orprocess the information. For example, network link 532 b may provide aconnection through local network 580 to a host computer 582 or toequipment 584 operated by an Internet Service Provider (ISP). ISPequipment 584 in turn provides data communication services through thepublic, world-wide packet-switching communication network of networksnow commonly referred to as the Internet 590. A computer called a server592 connected to the Internet provides a service in response toinformation received over the Internet. For example, server 592 providesrouting information for use with switching system 530.

The switching system 530 includes logic and circuitry configured toperform switching functions associated with passing information amongelements of network 580, including passing information received alongone network link, e.g. 532 a, as output on the same or different networklink, e.g., 532 c. The switching system 530 switches information trafficarriving on an input interface to an output interface according topre-determined protocols and conventions that are well known. In someembodiments, switching system 530 includes its own processor and memoryto perform some of the switching functions in software. In someembodiments, switching system 530 relies on processor 502, memory 504,ROM 506, storage 508, or some combination, to perform one or moreswitching functions in software. For example, switching system 530, incooperation with processor 504 implementing a particular protocol, candetermine a destination of a packet of data arriving on input interfaceon link 532 a and send it to the correct destination using outputinterface on link 532 c. The destinations may include host 582, server592, other terminal devices connected to local network 580 or Internet590, or other routing and switching devices in local network 580 orInternet 590.

The invention is related to the use of computer system 500 forimplementing the techniques described herein. According to oneembodiment of the invention, those techniques are performed by computersystem 500 in response to processor 502 executing one or more sequencesof one or more instructions contained in memory 504. Such instructions,also called software and program code, may be read into memory 504 fromanother computer-readable medium such as storage device 508. Executionof the sequences of instructions contained in memory 504 causesprocessor 502 to perform the method steps described herein. Inalternative embodiments, hardware, such as application specificintegrated circuit 520 and circuits in switching system 530, may be usedin place of or in combination with software to implement the invention.Thus, embodiments of the invention are not limited to any specificcombination of hardware and software.

The signals transmitted over network link 532 and other networks throughcommunications interfaces such as interface 570, which carry informationto and from computer system 500, are exemplary forms of carrier waves.Computer system 500 can send and receive information, including programcode, through the networks 580, 590 among others, through network links532 and communications interfaces such as interface 570. In an exampleusing the Internet 590, a server 592 transmits program code for aparticular application, requested by a message sent from computer 500,through Internet 590, ISP equipment 584, local network 580 and networklink 532 b through communications interface in switching system 530. Thereceived code may be executed by processor 502 or switching system 530as it is received, or may be stored in storage device 508 or othernon-volatile storage for later execution, or both. In this manner,computer system 500 may obtain application program code in the form of acarrier wave.

Various forms of computer readable media may be involved in carrying oneor more sequence of instructions or data or both to processor 502 forexecution. For example, instructions and data may initially be carriedon a magnetic disk of a remote computer such as host 582. The remotecomputer loads the instructions and data into its dynamic memory andsends the instructions and data over a telephone line using a modem. Amodem local to the computer system 500 receives the instructions anddata on a telephone line and uses an infra-red transmitter to convertthe instructions and data to an infra-red signal, a carrier wave servingas the network link 532 b. An infrared detector serving ascommunications interface in switching system 530 receives theinstructions and data carried in the infrared signal and placesinformation representing the instructions and data onto bus 510. Bus 510carries the information to memory 504 from which processor 502 retrievesand executes the instructions using some of the data sent with theinstructions. The instructions and data received in memory 504 mayoptionally be stored on storage device 508, either before or afterexecution by the processor 502 or switching system 530.

6.0 Extensions and Alternatives

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A method for protecting a packet switched network at a servicegateway, comprising the steps of: receiving user data at a gatewayserver in a communication path on a packet switched network between anetwork access server and a content server, wherein the user dataincludes subscriber identifier data that indicates a unique identifierfor a particular user, network address data that indicates a networkaddress for a host used by the particular user, NAS data that indicatesan identifier for the network access server, flow list data thatindicates one or more open data packet flows, wherein an open datapacket flow comprises a data packet that indicates a start of a seriesof data packets that pass through the gateway server for communicationsbetween the user and the content server without a correspondingtermination of the series of data packets between the user and thecontent server, and suspicious activity data that indicates a value fora property of the open data packet flows that indicates suspiciousactivity; determining whether an intrusion condition is satisfied basedon the suspicious activity data, wherein the intrusion condition isassociated with a scanning attack, which is identified through thesuspicious activity data by recognizing an excessive number of openflows being established per second such that other flows are deniedaccess because of the excessive number of open flows; and if it isdetermined that the intrusion condition is satisfied, then responding tothe intrusion by dropping packets originated from an Internet Protocol(IP) address associated with the particular end user and by sending amessage to remove the particular end user's IP address from a list ofauthorized users for the network; and communicating a message based onthe subscriber identifier data to a billing agent that is involved indetermining charges for a particular user for causing the billing agentto penalize the particular user.
 2. A method as recited in claim 1,wherein: the property that indicates suspicious activity is a number ofthe open data packet flows that are started within a particular periodof time; and said step of determining whether an intrusion condition issatisfied comprises determining whether the number of the open datapacket flows that are started within the particular period of timeexceeds a threshold rate.
 3. A method as recited in claim 1, wherein:the property that indicates suspicious activity is a number of the opendata packet flows for which there is no acknowledgement message from theuser; and said step of determining whether an intrusion condition issatisfied comprises determining whether the number of the open datapacket flows for which there is no acknowledgement message from the userexceeds at least one of a threshold number and a threshold percentage ofa total number of open data packet flows.
 4. A method as recited inclaim 1, wherein: the property that indicates suspicious activity is anumber of the open data packet flows for which there is noacknowledgement message from the content server; and said step ofdetermining whether an intrusion condition is satisfied comprisesdetermining whether the number of the open data packet flows for whichthere is no acknowledgement message from the content server exceeds atleast one of a threshold number and a threshold percentage of a totalnumber of open data packet flows.
 5. A method as recited in claim 1,said step of responding to the intrusion further comprising the step ofsending a message to the network access server based on the NAS data toterminate communications from the particular user.
 6. A method asrecited in claim 5, said step of sending the message to the networkaccess server further causing the network access server to freeresources allocated to the user in an access network between the userand the network access server.
 7. A method as recited in claim 6, saidstep of sending the message to the network access server comprising thestep of sending a Remote Access Dial-in User Service (RADIUS) Packet ofDisconnect (POD) to the network access server, whereby the networkaccess server tears down a communication path in the access network andfrees resources in the access network allocated to the user.
 8. A methodas recited in claim 1, wherein the billing agent causes a message thatrequests the particular user to cease undesired activity to be sent tothe particular user outside the packet-switched network.
 9. A method asrecited in claim 1, wherein the billing agent causes the particular userto lose authorization for access to a set of one or more contentservers.
 10. A method as recited in claim 1, wherein the billing causesan excess fee to be charged to the particular user.
 11. A method asrecited in claim 1, said step of responding to the intrusion furthercomprising the steps of: determining whether a logon message is receivedfor authorization of access for the packet-switched network, which logonmessage includes the subscriber identifier data; and if the logonmessage that includes the subscriber identifier data is received, thennot granting access for the packet-switched network.
 12. A method asrecited in claim 11, said step of not granting access for thepacket-switched network further comprising the step of not grantingaccess for the packet-switched network during a particular penaltyperiod of time.
 13. A method as recited in claim 1, said step ofresponding to the intrusion further comprising the steps of: determiningwhether a logon message is received for authorization of access for thepacket-switched network, which logon message includes the subscriberidentifier data; and if the logon message that includes the subscriberidentifier data is received, then forwarding the message to a particularserver to generate a special response for the particular user.
 14. Amethod as recited in claim 1, said step of responding to the intrusionfurther comprising the step of deleting all data flows associated withthe same subscriber identifier.
 15. The method as recited in claim 1,further comprising: receiving, at a billing agent server, malicioussubscriber data that includes subscriber identifier data that indicatesa unique identifier for a particular user and intrusion data thatindicates the particular user is associated with an intrusion on thetarget network, wherein the billing agent server is configured torespond to the malicious subscriber data by dropping packets originatedfrom an Internet Protocol (IP) address associated with the particularend user and by sending a message to remove the particular end user's IPaddress from a list of authorized users for the network; and in responseto receiving the malicious subscriber data, penalizing the particularuser by: sending an out-of-band message to the particular user to warnthe particular user to cease the intrusion on the target network; andcausing a penalty fee to be charged to an account associated with theparticular user.
 16. A method as recited in claim 15, furthercomprising: determining whether a penalty period has expired; and if itis determined that a penalty period has expired, then causing theparticular user to be restored to a list of authorized users of thetarget network.
 17. An apparatus for protecting a packet switchednetwork at a service gateway, comprising: means for receiving user dataat a gateway server in a communication path on a packet switched networkbetween a network access server and a content server, wherein the userdata includes subscriber identifier data that indicates a uniqueidentifier for a particular user, network address data that indicates anetwork address for a host used by the particular user, NAS data thatindicates an identifier for the network access server, flow list datathat indicates one or more open data packet flows, wherein an open datapacket flow comprises a data packet that indicates a start of a seriesof data packets that pass through the gateway server for communicationsbetween the user and the content server without a correspondingtermination of the series of data packets between the user and thecontent server, and suspicious activity data that indicates a value fora property of the open data packet flows that indicates suspiciousactivity; means for determining whether an intrusion condition issatisfied based on the suspicious activity data determining whether anintrusion condition is satisfied based on the suspicious activity data,wherein the intrusion condition is associated with a scanning attack,which is identified through the suspicious activity data by recognizingan excessive number of open flows being established per second such thatother flows are denied access because of the excessive number of openflows; and means for responding to the intrusion, if it is determinedthat the intrusion condition is satisfied, by dropping packetsoriginated from an Internet Protocol (IP) address associated with theparticular end user and by sending a message to remove the particularend user's IP address from a list of authorized users for the network;means for receiving, at a billing agent server, malicious subscriberdata that includes subscriber identifier data that indicates a uniqueidentifier for a particular user and intrusion data that indicates theparticular user is associated with an intrusion on the target network;and means for penalizing the particular user in response to receivingthe malicious subscriber data by causing the particular user to beremoved from a list of authorized users of the target network; sendingan out-of-band message to the particular user to warn the particularuser to cease the intrusion on the target network; and causing a penaltyfee to be charged to an account associated with the particular user. 18.An apparatus for protecting a packet switched network at a servicegateway server in a communication path on the packet switched networkbetween a network access server and a content server on the packetswitched network comprising: a network interface that is coupled to thepacket switched network for communicating therewith a data packet; oneor more processors; a computer-readable medium; and one or moresequences of instructions stored in the computer-readable medium, which,when executed by the one or more processors, causes the one or moreprocessors to carry out the step of: receiving user data that includessubscriber identifier data, network address data, NAS data, flow listdata and suspicious activity data, determining whether an intrusioncondition is satisfied based on the suspicious activity data; if it isdetermined that the intrusion condition is satisfied, then responding tothe intrusion by dropping packets originated from an Internet Protocol(IP) address associated with the particular end user and by sending amessage to remove the particular end user's IP address from a list ofauthorized users for the network, determining whether an intrusioncondition is satisfied based on the suspicious activity data, whereinthe intrusion condition is associated with a scanning attack, which isidentified through the suspicious activity data by recognizing anexcessive number of open flows being established per second such thatother flows are denied access because of the excessive number of openflows; communicating a message based on the subscriber identifier datato a billing agent that is involved in determining charges for aparticular user for causing the billing agent to penalize the particularuser; and wherein the subscriber identifier data indicates a uniqueidentifier for a particular user, the NAS data indicates an identifierfor the network access server, the flow list data indicates one or moreopen data packet flows, wherein an open data packet flow comprises adata packet that indicates a start of a series of data packets that passthrough the gateway server for communications between the user and thecontent server without a corresponding termination of the series of datapackets between the user and the content server, and the suspiciousactivity data indicates a value for a property of the open data packetflows that indicates suspicious activity.
 19. An apparatus as recited inclaim 18, wherein: the property that indicates suspicious activity is anumber of the open data packet flows that are started within aparticular period of time; and said step of determining whether anintrusion condition is satisfied comprises determining whether thenumber of the open data packet flows that are started within theparticular period of time exceeds a threshold rate.
 20. An apparatus asrecited in claim 18, wherein: the property that indicates suspiciousactivity is a number of the open data packet flows for which there is noacknowledgement message from the user; and said step of determiningwhether an intrusion condition is satisfied comprises determiningwhether the number of the open data packet flows for which there is noacknowledgement message from the user exceeds at least one of athreshold number and a threshold percentage of a total number of opendata packet flows.
 21. An apparatus as recited in claim 18, wherein: theproperty that indicates suspicious activity is a number of the open datapacket flows for which there is no acknowledgement message from thecontent server; and said step of determining whether an intrusioncondition is satisfied comprises determining whether the number of theopen data packet flows for which there is no acknowledgement messagefrom the content server exceeds at least one of a threshold number and athreshold percentage of a total number of open data packet flows.
 22. Anapparatus as recited in claim 18, said step of responding to theintrusion further comprising the step of sending a message to thenetwork access server based on the NAS data to terminate communicationsfrom the particular user.
 23. An apparatus as recited in claim 22, saidstep of sending the message to the network access server further causingthe network access server to free resources allocated to the user in anaccess network between the user and the network access server.
 24. Anapparatus as recited in claim 23, said step of sending the message tothe network access server comprising the step of sending a Remote AccessDial-in User Service (RADIUS) Packet of Disconnect (POD) to the networkaccess server, whereby the network access server tears down acommunication path in the access network and frees resources in theaccess network allocated to the user.
 25. An apparatus as recited inclaim 18, wherein the billing agent causes a message that requests theparticular user to cease undesired activity to be sent to the particularuser outside the packet-switched network.
 26. An apparatus as recited inclaim 18, wherein the billing agent causes the particular user to loseauthorization for access to a set of one or more content servers.
 27. Anapparatus as recited in claim 18, wherein the billing causes an excessfee to be charged to the particular user.
 28. An apparatus as recited inclaim 18, said step of responding to the intrusion further comprisingthe steps of: determining whether a logon message is received forauthorization of access for the packet-switched network, which logonmessage includes the subscriber identifier data; and if the logonmessage that includes the subscriber identifier data is received, thennot granting access for the packet-switched network.
 29. An apparatus asrecited in claim 28, said step of not granting access for thepacket-switched network further comprising the step of not grantingaccess for the packet-switched network during a particular penaltyperiod of time.
 30. An apparatus as recited in claim 18, said step ofresponding to the intrusion further comprising the steps of: determiningwhether a logon message is received for authorization of access for thepacket-switched network, which logon message includes the subscriberidentifier data; and if the logon message that includes the subscriberidentifier data is received, then forwarding the message to a particularserver to generate a special response for the particular user.
 31. Anapparatus as recited in claim 18, said step of responding to theintrusion further comprising the step of deleting all data flowsassociated with the same subscriber identifier.
 32. An apparatus forresponding to intrusions on a packet switched network at a billing agentserver that provides payment information for a remote user who accessesthe packet switched network, comprising: a network interface that iscoupled to the packet switched network for communicating therewith adata packet; one or more processors, a computer-readable medium; and oneor more sequences of instructions stored in the computer-readablemedium, which, when executed by the one or more processors, causes theone or more processors to carry out the step of: receiving, from asubscriber-aware service gateway server, malicious subscriber data thatincludes subscriber identifier data that indicates a unique identifierfor a particular user and intrusion data that indicates the particularuser is associated with an intrusion on the packet switched network; andin response to receiving the malicious subscriber data, penalizing theparticular user by: causing the particular user to be removed from alist of authorized users of the packet switched network; droppingpackets originated from an Internet Protocol (IP) address associatedwith the particular end user; sending an out-of-band message to theparticular user to warn the particular user to cease the intrusion onthe packet switched network; determining whether a penalty period hasexpired; and if it is determined that a penalty period has expired, thencausing the particular user to be restored to a list of authorized usersof the target network.
 33. A non-transitory computer-readable mediumcarrying one or more sequences of instructions for protecting a packetswitched network at a service gateway, wherein execution of the one ormore sequences of instructions by one or more processors, included in anetwork element configured to send and to receive Internet protocol (IP)packets, causes the one or more processors to perform the steps of:receiving user data at a gateway server in a communication path on apacket switched network between a network access server and a contentserver, wherein the user data includes subscriber identifier data thatindicates a unique identifier for a particular user, network addressdata that indicates a network address for a host used by the particularuser, NAS data that indicates an identifier for the network accessserver, flow list data that indicates one or more open data packetflows, wherein an open data packet flow comprises a data packet thatindicates a start of a series of data packets that pass through thegateway server for communications between the user and the contentserver without a corresponding termination of the series of data packetsbetween the user and the content server, and suspicious activity datathat indicates a value for a property of the open data packet flows thatindicates suspicious activity; determining whether an intrusioncondition is satisfied based on the suspicious activity data determiningwhether an intrusion condition is satisfied based on the suspiciousactivity data, wherein the intrusion condition is associated with ascanning attack, which is identified through the suspicious activitydata by recognizing an excessive number of open flows being establishedper second such that other flows are denied access because of theexcessive number of open flows; and if it is determined that theintrusion condition is satisfied, then responding to the intrusion bydropping packets originated from an Internet Protocol (IP) addressassociated with the particular end user and by sending a message toremove the particular end user's IP address from a list of authorizedusers for the network; receiving, at a billing agent server, malicioussubscriber data that includes subscriber identifier data that indicatesa unique identifier for a particular user and intrusion data thatindicates the particular user is associated with an intrusion on thetarget network; and in response to receiving the malicious subscriberdata, penalizing the particular user by: causing the particular user tobe removed from a list of authorized users of the target network; anddropping packets originated from an Internet Protocol (IP) addressassociated with the particular end user.
 34. A computer-readable mediumas recited in claim 33, wherein execution of the code further causes theone or more processors to perform the steps of: determining whether apenalty period has expired; and if it is determined that a penaltyperiod has expired, then causing the particular user to be restored to alist of authorized users of the target network.